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Re: [PATCH 2/6] x86/MPX: fix address size handling
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Jan Beulich <JBeulich at suse dot com>
- Cc: kirill dot yukhin at intel dot com, Binutils <binutils at sourceware dot org>
- Date: Tue, 8 Oct 2013 08:32:02 -0700
- Subject: Re: [PATCH 2/6] x86/MPX: fix address size handling
- Authentication-results: sourceware.org; auth=none
- References: <5254349502000078000F9A3D at nat28 dot tlf dot novell dot com> <525435BC02000078000F9A51 at nat28 dot tlf dot novell dot com> <CAMe9rOrZuQRgcaUNgex=MDFjKYxsJimK1aCMqzEyM2XbZXCo4w at mail dot gmail dot com> <52543EE302000078000F9AF4 at nat28 dot tlf dot novell dot com>
On Tue, Oct 8, 2013 at 8:20 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> On 08.10.13 at 17:15, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>> On Tue, Oct 8, 2013 at 7:41 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>> While address overrides are ignored in 64-bit mode (and hence shouldn't
>>> result in an error), trying to use 16-bit addressing is documented to
>>> result in #UD, and hence the assembler should reject the attempt.
>>>
>>> gas/
>>> 2013-10-08 Jan Beulich <jbeulich@suse.com>
>>>
>>> * tc-i386.c (md_assemble): Alter address size checking for MPX
>>> instructions.
>>>
>>> --- 2013-10-07/gas/config/tc-i386.c
>>> +++ 2013-10-07/gas/config/tc-i386.c
>>> @@ -3549,10 +3549,15 @@ md_assemble (char *line)
>>> if (i.bnd_prefix && !i.tm.opcode_modifier.bndprefixok)
>>> as_bad (_("expecting valid branch instruction after `bnd'"));
>>>
>>> - if (i.tm.cpu_flags.bitfield.cpumpx
>>> - && flag_code == CODE_64BIT
>>> - && i.prefix[ADDR_PREFIX])
>>> - as_bad (_("32-bit address isn't allowed in 64-bit MPX instructions."));
>>
>> It is done on purpose. When 32-bit address prefix in 64-bit is ignored,
>> MPX doesn't work correctly for x32.
>
> I don't understand: It _is_ being ignored by the hardware as per
> the documentation. So x32 need to get along with that. Maybe
> an example would help, so I could understand why you think
> this _needs_ to be an error...
>
X32 won't work with MPX since hardware assumes pointer
size is always 64 bit in 64-bit mode with or without address
size prefix. MPX depends on correct pointer size to work.
I don't want people to use MPX in x32 by accident.
--
H.J.